A Capacitor Voltage Balancing Method With Fundamental Sorting Frequency for Modular Multilevel Converters Under Staircase Modulation

A fundamental frequency-sorting algorithm with staircase modulation is proposed to balance the floating capacitors for modular multilevel converters. The driving pulses are assigned to the submodules at every fundamental period according to their charging capabilities for the capacitors. The charging capabilities of the driving pulses can be evaluated by sorting the voltage increments of the capacitors or derived from the symmetrical characteristic of the curve between the voltage increments and the pulse numbers. With this method, all the power devices switch only once per fundamental period, which is suitable for high-power applications. Meanwhile, the sorting frequency decreases to the fundamental frequency. Hence, a large number of calculation resources can be saved. Moreover, it does not need to measure the arm currents so that several current sensors can be saved and the communication protocol between the central and local controllers can be simplified. At last, a three-phase simulation platform with 20 submodules per arm and a down-scaled experimental prototype with eight submodules in each arm are built to validate the proposed voltage-balancing method.